The present invention relates to an apparatus for detecting an input signal comprising means for comparing the input signal with threshold values.
FIG. 1 is a circuit diagram showing a known typical apparatus for detecting or restoring a bivalent signal. An input signal is supplied to an input terminal 1 and is supplied to positive and negative input terminals of comparators 2 and 3, respectively to the other inputs of which positive and negative threshold values are applied from fixed reference voltage sources 4 and 5, respectively. Then output signals representing positive and negative input signals are derived at output terminals 6 and 7, respectively.
In the known apparatus, when the input signal having a base line 8 which fluctuates greatly over peaks of a signal 9 as shown in FIG. 2A is supplied to the input terminal 1, the input signal could not be obtained correctly unless the positive and negative threshold values 10 and 11 are set beyond a fluctuation range of the base line 8. Then the signals which do not exceed the threshold values 10 and 11 are not detected as illustrated by dotted lines in FIGS. 2B and 2C. FIG. 2B shows detected signals greater than the base level 8 and FIG. 2C those smaller than the base level. In this manner, the known apparatus could not correctly detect the input signal.
In order to avoid such a drawback, it has been proposed to pass the input signal through a high pass filter before comparing it with the threshold levels. However, in such a case it is quite difficult to remove only the fluctuation of the base line, particuarly when the base line contains pulsatory noise.